ID this substance

[JuniorMonsterKeeper]

I recently had to clean out myquarantine tank and set it to recycle which it’s been doing for a few weeks now. I placed a decently sized stone from my 125 cichlid bichir tank in the tank to help seed the new sponge filter with beneficial bacteria. I left the tank lights off but as this tsnk sits under my 125 in my dining room which is off from my living room there is a decent amount of ambient light that enter the tank through the day. But I have this substance that is sitting on the bottom of my tank and stone and am not sure what it could be.

Please help me Id this and let me know if I should be concerned

 

[FishEZ]

It's Mulm. Just siphon it out. Raise the temp a little and eventually it will stop forming.